Television receivers are available in a wide variety of sizes and capabilities, including picture quality. As picture quality and/or picture display size increases, the cost of the television receiver increases. While there are some portions of a television receiver which are essential to detect and decode the picture for display and separate the sound for reproduction, there are other portions of the receiver (herein referred to as a "decoder") which, if present, enhance overall performance thereof.
It is therefore most useful to provide a decoder for a television receiver which is comprised of plural modules, at least some of which are entirely optional to satisfactory picture display. In one minimum, very inexpensive configuration, for example, the decoder may include just enough signal processing equipment to provide a minimum, yet adequate picture quality. Such a decoder might be appropriate for a television receiver having a small picture display size, such as a fourteen inch diagonal dimension. By simply adding modules to the decoder, a very high quality display would be provided for a large picture display size, such as 50 inches.
It is known that the visual appearance of a television display, particularly that of a large dimension display, is enhanced by doubling the number of horizontal scanning lines. In large dimension displays, the line scan structure becomes a visible and objectionable picture artifact which distracts from the aesthetic quality of the picture display. As the display format is reduced, the line scan structure becomes less objectionable, but can be seen, particularly upon high contrast, high quality displays, such as the Sony Trinitron.TM. display. By doubling the number of scanning lines at the receiver, the line scan structure becomes much less visible.
Scan line doubling is only one enhancement of a plurality of known enhancements which may be applied at the television receiver in order to improve the quality of the picture display. Noise reduction and crispening are also known enhancements.
A scan line doubler generates additional scan lines, based upon the picture content of the original scan lines. Line averaging is a known approach. With line averaging, a new pixel is generated as an average of a pixel value at the same spatial region or area of two adjacent scan lines A and B. The resultant average pixel, typically (A+B)/2, is then placed on a new line generated in the space between the two original lines A and B.
There are known drawbacks to spatial averaging within line doublers. First, there is a loss of vertical and diagonal resolution. Second, there is a resultant flickering, which becomes very perceptible at high transitional levels, such as those above 50 IRE units. These problems are addressed and a very satisfactory solution is presented by the disclosure contained in a commonly assigned U.S. Pat. No. 4,989,090, entitled "Television Scan Line Doubler Including Temporal Median Filter", the disclosure of which is hereby incorporated by reference. However, the scan line doubler described in the referenced U.S. Patent did not lend itself to low cost implementation for the television consumer marketplace, and the temporal dimension processing was not modularized or "optional", thereby providing expansibility of the processing capability at the television decoder. The present invention can be considered to be a significant improvement in aspects of the approach taken in the above-referenced U.S. Patent.